The present disclosure relates, in various embodiments, to imaging members comprising an overcoat layer and methods of forming such imaging members. The overcoat layer in accordance with the present disclosure is a crosslinked polymer matrix which binds polytetrafluoroethylene nanoparticles.
In the art of electrophotography, an electrophotographic imaging member or plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation, for example light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles, for example from a developer composition, on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photosensitive members.
Electrophotographic imaging members are usually multilayered photoreceptors that comprise a substrate support, an electrically conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, and an optional protective or overcoat layer(s). The imaging members can take several forms, including flexible belts, rigid drums, etc. For most multilayered flexible photoreceptor belts, an anti-curl layer is usually employed on the back side of the substrate support, opposite to the side carrying the electrically active layers, to achieve the desired photoreceptor flatness.
Imaging members are generally exposed to repetitive electrophotographic cycling which subjects exposed layers of imaging devices to abrasion, chemical attack, heat and multiple exposures to light. This repetitive cycling leads to a gradual deterioration in the mechanical and electrical characteristics of the exposed layers. For example, repetitive cycling has adverse effects on exposed portions of the imaging member. Attempts have been made to overcome these problems. However, the solution of one problem often leads to additional problems.
In electrophotographic imaging devices, the charge transport layer may comprise a high loading of a charge transport compound dispersed in an appropriate binder. The charge transport compound may be present in an amount greater than about 35% based on the weight of the charge transport layer. For example, the charge transport layer may comprise 50% of a charge transport compound in about 50% binder. A high loading of charge transport compound appears to drive the chemical potential of the charge transport layer to a point near the metastable state, which is a condition that induces crystallization, leaching and stress cracking when placed in contact with a chemically interactive solvent or ink. Photoreceptor functionality may be completely destroyed when a charge transport layer having a high loading of a charge transport molecule is contacted with liquid ink. It is thus desirable to eliminate charge transport molecule crystallization, leaching and solvent-stress charge transport layer cracking.
Cracks developed in the charge transport layer during cycling are a frequent phenomenon and most problematic because they can manifest themselves as print-out defects which adversely affect copy quality. Charge transport layer cracking has a serious impact on the versatility of a photoreceptor and reduces its practical value for automatic electrophotographic copiers, duplicators and printers.
Another problem encountered with electrophotographic imaging members is corona species induced deletion in print due to degradation of the charge transport molecules by chemical reaction with corona species. During electrophotographic charging, corona species are generated. Corona species include, for example ozone, nitrogen oxides, acids and the like.
Other problems affecting the performance of the imaging member include lateral charge migration and stress cracking in the photoreceptor. The concentration of charge transport molecules in the charge transport layer is a known factor affecting the degree of lateral charge migration. In particular, higher concentrations of charge transport molecules near the surface of the charge transport layer tend to result in a higher degree of lateral charge migration and more stress cracks.
Recent developments in reducing these defects have been directed to the formation of the charge transport layer. For example, the charge transport layer may be coated in two passes with a high loading of charge transport material in the first pass and a decreased loading of charge transport material in the second pass. The above two-pass coating should provide a charge transport layer with sufficient concentration of charge transport material for effective charge transport that has a reduced concentration of charge transport materials at the surface of the charge transport layer. The theoretical benefit of a lower concentration of charge transport material in the second pass is not completely achieved because the first layer tends to dissolve during coating of the second layer. This results in intermixing of the first pass and second pass layers and a greater concentration of charge transport material closer to the surface of the charge transport layer than is theoretically expected.
One approach to achieving longer photoreceptor drum life is to form a protective overcoat on the imaging surface, e.g. the charge transporting layer of a photoreceptor. This overcoat layer must satisfy many requirements, including transporting holes, resisting image deletion, resisting wear and avoidance of perturbation of underlying layers during coating.
There are many factors which go into making an effective, wear resistant overcoat layer. One factor is pot life. Since the drums are typically dip coated, one of the requirements for the overcoat material is ease and economical synthesis of materials and a coating solution pot life of several weeks. Pot life is the life of the coating composition without changes in its properties so that the same mixture can be used for several weeks. With coating compositions that ultimately cross-link and provide wear protection, there is a danger of initiation of cross-linking in the pot itself rendering the remaining material in the pot useless. Since the unused material must be discarded and the pot cleaned or replaced, this waste of material and effort has a significant negative impact on the manufacturing cost. In addition, the overcoat layer must be compatible with and adhere well to the charge transport layer on which it is placed. It must also have necessary electrical properties; it should transport holes, yet resist image deletion.
Because of these factors, as well as cost, most overcoat layers are generally very thin. They are usually less than 5 mm thick.
One of the toughest known overcoats includes cross-linked polyamide (e.g. Luckamide) containing N,N′-diphenyl-N,N′-bis(3-hydroxyphenyl)-[1,1′-biphenyl]-4,4′-diamine. This overcoat is described in U.S. Pat. No. 5,368,967, the entire disclosure of which is incorporated herein by reference. Durable photoreceptor overcoats containing cross-linked polyamide (e.g. Luckamide) and N,N′-diphenyl-N,N′-bis(3-hydroxyphenyl)-[1,1′-biphenyl]-4,4′-diamine (DHTBD) have been prepared using oxalic acid and trioxane to improve photoreceptor life by at least a factor of 3 to 4. The improved wear resistance involved cross-linking of Luckamide under heat treatment, e.g. 110° C.-120° C. for about 30 minutes. However, adhesion of this overcoat to certain photoreceptor charge transport layers, containing certain polycarbonates (e.g., Z-type 300) and charge transport materials [e.g., bis-N,N-(3,4-dimethylphenyl)-N-(4-biphenyl) amine and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine] is greatly reduced under such drying conditions. On the other hand, under drying conditions of below about 110° C., the overcoat adhesion to the charge transport layer was good, but the overcoat had a high rate of wear. Thus, there was an unacceptably small drying condition window for the overcoat to achieve the targets of both adhesion and wear rate.
Other known overcoats are formed from hydrolyzed siloxane gel or FX silicones. Unfortunately, these overcoats are very costly. In addition, they are relatively incompatible with most charge transport layers and do not adhere well to them.
Nano-size particles have been successfully used to reinforce polymer materials. For example, polytetrafluoroethylene (PTFE) nanoparticles have been used in organic photoreceptors. However, PTFE has low compatibility with other polymer binders like polycarbonate, which leads to unstable dispersion distribution leading to non-uniform morphology of the overcoat and large-scale particle aggregation.
There is a need for a relatively thick overcoat layer that will reduce lateral charge migration, deletion, and/or stress cracking in an imaging member while still providing an imaging member that exhibits satisfactory electrical properties.